This invention relates to bulb generators and motors and more particularly to a stator yoke construction for horizontal and semi-horizontal bulb generators and motors.
Typically, the total bulb unit of a bulb generator or motor consists of a nose and intermediate ring section on the upstream side of the stator and a turbine cone on the downstream side of the stator. The stator yoke is surrounded by water and is part of the total bulb unit. The yoke of the bulb generator or motor has to satisfy a number of structural requirements. For example, the yoke has to be rigid enough to withstand forces due to the magnetic field and the external hydrostatic pressure both at rated and transient conditions without significant deformation. The yoke of a bulb generator typically contains a cylindrical wrapping plate and yoke plates. The yoke plates together with the wrapping plate represents a circular beam which is subjected to radial bending due to the magnetic field and the hydrostatic pressure. The magnetic field based in the rotor poles produces a sinusoidal deformation wave with a node number equal to the pole number. The external hydrostatic pressure caused by the surrounding water is dependent upon the depth under the surface of the water. Therefore, the pressure is greater at the bottom region of the yoke than at the top region of the yoke. This uneveness of pressure causes the yoke to be deformed in an elliptical or egg shape. Quite understandably, deformation into an egg shape is highly undesirable because the deformation interferes with the tolerances of the parts which rotate with respect to one other. Accordingly, it will be appreciated that it would be highly desirable to provide a stator yoke for a bulb generator which resists deformation.
There are also other types of loads and forces encountered during normal operation. For example, there are magnetic forces encountered if the stator and rotor are not concentric, torsional forces due to the rated torque and various axial hydrostatic loads. These loads are typically changing during transient conditions and the yoke has to satisfy all the combinations of static and dynamic forces. An important consideration in yoke design is the maintenance of the radial air gap between the rotor and stator. A change in the air gap or uneveness of the air gap around the rotor has a self amplifying effect which is undesirable. A reduction of air gap increases the magnetic forces which produces a further reduction of the air gap. Actually, it is not the stresses which are the main concern in yoke design; rather, it is the deformation and stability.
Since bulb generators have a relatively small air gap, these considerations are more important for bulb generators than other hydrogenerators or motors. In order to limit the air gap changes, the radial bending stiffness of the yoke has to be as large as possible. This is usually difficult to satisfy because the hydrodynamical and electrical requirements are in conflict. Viewed hydrodynamically, the highest possible efficiency would require the smallest possible yoke outer diameter. An efficient generator should have a large stator diameter. Since the radial yoke height has to shrink between the increasing stator outer diameter and the decreasing yoke outer diameter, the necessary bending stiffness cannot be achieved.
The normal way to solve the conflict is to transfer the missing stiffness to components outside of the central part of the yoke. For example, the stiffness may be transferred to the flanges at the ends of the wrapping or to the bridges and brackets supporting the bearings, coolers, brakes, etc. This transfer of radial rigidity depends on the axial bending stiffness of the wrapping and axial stiffener ribs. If the axial stiffness is low, the flanges and brackets are not able to help. Therefore, the thickness of the wrapping plate and ribs has to be increased considerably. Many axial ribs have to be welded in between the flanges and yoke plates, increasing the manufacturing costs and weight. Accordingly, it would be appreciated that it would be highly desirable to provide a yoke for a bulb generator which has the strength required to resist deformation and changes in the air gap caused by static and dynamic forces which is economical to manufacture.
The axial cooling air flow between the wrapping plate and the core represents a further problem. The yoke plates typically must be equipped with many large openings providing sufficient cross-sectional area for cooling air flow. However, these holes weaken the yoke and reduce the rigidity. Accordingly, it would be appreciated that it would be highly desirable to provide a yoke which has sufficient cooling air flow without a reduction in the strength and rigidity of the yoke.
It is an object of the present invention to provide a yoke for a bulb generator which has sufficient rigidity and stability to resist changes in the radial air gap between the rotor and stator.
Another object of this invention is to provide a stator yoke which has the necessary bending stiffness and axial bending strength to reduce deformation caused by static and dynamic forces.
Another object of the invention is to provide a stator yoke for a bulb generator which has sufficient flow cross section for effective cooling while maintaining the rigidity of the stator yoke.
Yet another object of the present invention is to provide a stator yoke construction for a bulb generator which is economical to manufacture and which has reduced weldments and improved flow conditions for effective cooling.